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Introduction to Node.js

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The node Command

We can execute Node.js programs in the terminal by typing the node command, followed by the file name.

The example command runs app.js, assuming there is a file with that name in the current working directory.

node app.js

Node.js REPL

Node.js comes with REPL, an abbreviation for read–eval–print loop. REPL contains three different states:

  • a read state where it reads the input from a user,
  • an eval state where it evaluates the user’s input
  • a print state where it prints out the evaluation to the console

After these states are finished, the REPL loops through these states repeatedly. REPL is useful as it gives back immediate feedback which can be used to perform calculations and develop code. 

$ node
Welcome to Node.js v22.19.0.
> console.log("HI")
HI

Node.js Global Object

The Node.js environment has a global object that contains every Node-specific global property. The global object can be accessed by executing console.log(global), or simply global in the terminal with the REPL running. In order to see just the keys, use Object.keys(global) instead.

Since global is an object, new properties can be assigned to it via global.name_of_property = 'value_of_property'. 

// access global within a script
console.log(global);


// add a new property to global
global.car = 'DeLorean';

Node.js Process Object

A process is the instance of a computer program that is being executed. Node has a global process object with useful properties. One of these properties is .env, which stores and controls information about the environment.

console.log(process.env.PWD); // Prints: /path/where/terminal/command/executed


if (process.env.NODE_ENV === 'development') {
  startDevelopmentServer();
  console.log('Testing! Testing! Does everything work?');
} else if (process.env.NODE_ENV === 'production') {
  startProductionServer();
}

Node.js process.argv

process.argv is a property that holds an array of the command-line values that were provided when the current process was initiated. The first element in the array is the absolute path to the Node installation, followed by the path to the file that was executed, and then any additional command-line arguments provided. 

// Command line values: node web.js testing several features
console.log(process.argv[2]); // 'testing' will be printed

Node.js process.memoryUsage()

process.memoryUsage() is a method that can be used to return information on the CPU demands of the current process. Here, “heap” refers to a pool of computer memory, rather than the data structure of the same name.

Using process.memoryUsage() will return an object in a format like the example given.

Node.js Modules

In Node.js, files are called modules. Modularity is a technique where a single program has distinct parts, each providing a single piece of the overall functionality, like pieces of a puzzle coming together to complete a picture. require() is a function used to import one module into another. 

const baseball = require('./babeRuth.js')

Node.js Core Modules

Node has several modules included within the environment to efficiently perform common tasks. These are known as the core modules. The core modules are defined within Node.js’s source and are located in the lib/ folder. A core module can be accessed by passing a string with the name of the module into the require() function.

const util = require('util');

Node.js Local Modules

In Node.js files are considered modules. Modules that are created locally are called local modules. These local modules are held in an object called module. This object has a property called exports which allows a module to be accessed in a different module. 

// type.js
// by using the export property we can use this module in another file
module.exports = class key {
	constructor(car) {
    this.car = car;
  }
};


// qwerty.js
// by requiring the type.js file we can we use the module in the type.js file
let Dog = require('./type.js');

Node Package Manager

NPM stands for node-package-manager. An NPM is essentially a collection of code from other developers that we can use. When Node is installed the npm command-line tool is downloaded as well. This command-line tool enables us to interact with the registry via our terminal.

The events Module

Node.js has an EventEmitter class that can be accessed by importing the events core module. Each event emitter instance has an .on() method that assigns a listener callback function to a named event. EventEmitter also has an .emit() method, which announces that a named event has occurred.

// Require in the 'events' core module
let events = require('events');


// Create an instance of the EventEmitter class
let myEmitter = new events.EventEmitter();
let showNewUser = (data) => {
 console.log(`participant: ${data}.`);
};


// Assign the showNewUser function as the listener callback for 'new user' events
myEmitter.on('new user', showNewUser)


// Emit a 'new user' event
myEmitter.emit('new user', 'Lily Pad') // Prints: participant: Lily Pad.

Asynchronous Node.js

Node.js is a non-blocking, asynchronous environment. The event loop in Node.js enables asynchronous actions to be handled in a non-blocking way. Node.js provides APIs which allow operations to be put in a queue, waiting to be executed after the previous operation finishes. If synchronous tasks never end, operations waiting in the event-queue will never execute.

let endgame = () => {
  console.log('I am inevitable')
};


// endgame will run after 1000ms
setTimeout(endgame, 1000);

The error Module

Many asynchronous Node.js APIs assume that the provided callback function will have an error passed as the first argument, followed by the resulting data of a (successful) operation. If the asynchronous task results in an error, the error will be passed in as the first argument to the callback function, with no second argument. If no error was thrown, then the first argument will be undefined, and the resulting data of the operation will be passed as the second argument.

Input/Output

Input is data that is given to the computer, while output is any data or feedback that a computer provides. In Node, we can get input from a user using the stdin.on() method on the process object. We are able to use this because process.stdin is an instance of EventEmitter. To give an output, we can use the stdout.write() method on the process object as well. This is because console.log() is actually a thin wrapper on stdout.write(). 

// recieve an input
process.stdin.on('data', onDataCallback);


// produce an output
process.stdout.write(data);

The fs Module

The file system controls how data on a computer is stored and retrieved. Node.js provides the fs core module, which allows interaction with the file system. Each method provided through the module has synchronous and asynchronous versions to allow for flexibility. One method available in the module is .readFile(), which reads data from the provided file.

const fs = require('fs');


// First argument is the file path
// The second argument is the file’s character encoding
// The third argument is an error-first callback function
fs.readFile('./file.txt', 'utf-8', callbackFunction);

Web Server

Node was designed with back end development needs as a top priority. One of these needs is the ability to create web servers. A web server is a computer process that listens for requests from clients and returns responses. A Node core module designed to meet these needs is the http module. This module has functions that simplify receiving and responding to requests.

const http = require('http');

Creating A Server

http.createServer() is a method that returns an instance of an http.server. The method .listen() in http.server tells the server to “listen” for incoming connections. We give http.createServer() a callback function also known as the requestListener, which will be triggered once the server is listening and receives a request. The requestlistener requests a request object and a response object.

// required in the http core module.
const http = require('http');


let requestListener = (request, response) => {
 	// code to be filled in depending on server
};


// assigning return value
const server = http.createServer(requestListener);


// assigning server port
server.listen(3000);

Readable/Writable Streams

In most cases, data is not processed all at once, but rather piece by piece, which we call a stream. Streaming data is preferred as it doesn’t require tons of RAM and doesn’t need to have all the data on hand to begin processing it. To read files line by line, we can use the .createInterface() method from the readline core module in conjunction with fs.createReadStream(). We can create a writeable stream by using the fs.createWriteStream() method, and write to the stream using .write().

const fs = require('fs');
const readline = require('readline');


// Readable stream
readline.createInterface({
  input: fs.createReadStream('text.txt')
});


// Writable Stream
const writableStream = fs.createWriteStream('output.txt');
writableStream.write('data');
writableStream.close();

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      30 hours